Oil sand deposits such as those found in the Athabasca Region of Alberta, Canada, generally comprise water-wet sand grains held together by a matrix of viscous heavy oil or bitumen. Bitumen is a complex and viscous mixture of large or heavy hydrocarbon molecules which contain a significant amount of sulfur, nitrogen and oxygen. Oil sands processing involves extraction and froth treatment to produce diluted bitumen which is further processed to produce synthetic crude oil and other valuable commodities. Extraction is typically conducted by mixing the oil sand in hot water and aerating the resultant slurry to promote the attachment of bitumen to air bubbles, creating a lower-density bitumen froth which floats and can be recovered in a primary separation vessel or “PSV”. Such bitumen froth is generally referred to as “primary bitumen froth”. Sand grains sink and are concentrated in the bottom of the PSV. They leave the bottom of the vessel as a wet tailings stream containing a small amount of bitumen. Middlings, a watery mixture containing fine solids and bitumen, extend between the froth and sand layers. The wet tailings and middlings are separately withdrawn, may be combined and sent to a secondary flotation process. This secondary flotation process is commonly carried out in a deep cone vessel (a “TOR” vessel) wherein air is sparged into the vessel to assist with flotation. The bitumen recovered by flotation in the TOR vessel is generally referred to as “secondary bitumen froth” and may be recycled to the PSV. The middlings from the deep cone vessel may be further processed in induced air flotation cells to recover contained bitumen.
Froth treatment is the process of reducing water and solids contents from the bitumen froths produced by the PSV, TOR vessel, etc. to produce a clean bitumen product (i.e., “diluted bitumen”) for downstream upgrading processes. It has been conventional to dilute this bitumen froth with a light hydrocarbon diluent, for example, with naphtha, to increase the difference in specific gravity between the bitumen and water and to reduce the bitumen viscosity, to thereby aid in the separation of the water and solids from the bitumen. This diluent diluted bitumen froth is commonly referred to as “dilfroth.” It is desirable to “clean” dilfroth, as both the water and solids pose fouling and corrosion problems in upgrading refineries. By way of example, the composition of naphtha-diluted bitumen froth typically might have a naphtha/bitumen ratio of 0.65 and contain 20% water and 7% solids. It is desirable to reduce the water and solids content to below about 3% and about 1%, respectively. Separation of the bitumen from water and solids is conducted by adding naphtha to the dilfroth and treating the dilfroth in a sequence of scroll and disc stack centrifuges.
A disc stack centrifuge separates bitumen from water and solids using extremely high centrifugal forces. When the heavy phase (i.e., water and solids) is subjected to such forces, the water and solids are forced outwards against the periphery of the rotating centrifuge bowl, while the light phase (i.e., bitumen) forms concentric inner layers within the bowl. Plates (i.e., the disc stack) provide additional surface settling area, which contributes to speeding up separation.
The oil-water interface or “E-line” is the boundary between the heavy and light phases. The position of the E-line may be varied in order to ensure that the separation takes place with maximum efficiency. However, if the E-line is positioned too far into the disc stack, it will increase wear and product quality will suffer from high solids and water content. Product solids lead to increased wear of downstream equipment, higher maintenance costs, and unplanned outages. If the E-line is positioned too far to the periphery of the centrifuge bowl, there is a risk of losing the E-line. When the E-line is lost, the feed exits through the heavy phase outlet tubes and centrifuge nozzles and results in oil and naphtha being lost to tailings, negatively impacting production and the environment.
There is currently no available technology which accurately measures the E-line position. E-line position must be inferred by measuring process variables. An e-line loss event can be verified by sampling the heavy phase and water streams to detect the presence of hydrocarbons. Instruments such as flow meters are often unreliable, providing poor quality or no data, unreliable readouts, and readings beyond normal range or including extraneous noise.
To avoid E-line loss, the centrifuge is typically run conservatively, with the E-line positioned well into the disc stack. However, this may result in poor product quality, with high water and solids contents in the diluted bitumen product. A computer-generated alarm is currently used to signal E-line loss but is unreliable, failing to detect about 50% of actual E-line loss events and generating over 95% false positives. When the E-line loss is not acted upon immediately, a significant hydrocarbon loss to tailings occurs.